The present invention is based on Japanese Patent Application No. 2000-200897, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a so-called tandem-type color image forming apparatus.
2. Description of the Related Art
So-called tandem-type color image forming apparatuses are conventionally known to have a plurality of image forming units containing toner of different colors arranged along an intermediate transfer belt as disclosed, for example, in Japanese Laid-Open Patent No. H7-28294.
In this type image forming apparatus, image forming units form an image on an intermediate transfer belt, and this formed image is printed on a sheet of a size desired by a user. The intermediate transfer belt comprises conductive carbon particles dispersed in resin so as to have semi conductivity.
When images are formed in the same range of the intermediate transfer belt and transferred to paper many times, the degree of dispersion (dispersion state) of the carbon particles in this range changes such that the resistance value and surface condition of the intermediate transfer belt changes in the area between the image area of repeated electrical current flow and the image area without electrical current flow. Image drift may be generated when an image is formed in the intermediate transfer belt range including the border location of the condition-changed areas.
An object of the present invention is to provide an image forming apparatus which prevents image drift due to change-of-state of the intermediate transfer member.
The color image forming apparatus of the present invention comprises an intermediate transfer member having a movable surface, a plural image forming units for forming superposed toner images on the intermediate transfer member at a predetermined image forming pitch, and a transfer member for transferring the superposed toner images formed on the intermediate transfer member onto a sheet of paper, wherein the peripheral length of the intermediate transfer member is a non-integral multiple of the image forming pitch.
In the image forming apparatus of the present invention, since the length of the intermediate transfer member is a non-integral multiple of the image forming pitch, the image forming range on the intermediate transfer member formed at a predetermined image forming pitch by each image forming unit shifts with each single rotation of the intermediate transfer member. In this way the image forming range can be dispersed on the intermediate transfer member, the range of the state-changed part of the intermediate transfer member caused by repeated image formation in the same range of the intermediate transfer member can be dispersed, and image drift on the sheet to which the image is transferred can be prevented. The state-change of the intermediate transfer member is the change of the surface condition and the resistance vale of the intermediate transfer member. The image forming pitch is desirably a length that matches the maximum sheet length and image interval distance. Furthermore, the intermediate transfer member desirably has semi conductivity.